The present invention is related to an optical system, a defect inspecting method and a defect inspecting apparatus with employment of this optical system, and a utilizing method for effectively utilizing inspection information, while the optical system is employed so as to inspect and observe defects and extraneous-material defects of patterns formed on substrates by way of thin-film manufacturing processes which are typically known in semiconductor manufacturing steps and manufacturing steps of flat panel displays.
To detect defects of very fine patterns formed on substrates by way of thin-film manufacturing processes, images having high image quality are necessarily required, the focuses and contrasts of which have been adjusted in high precision.
In the defect inspection field for instance, JP-A-2000-323542 discloses the image detecting method of objects as the conventional technique capable of acquiring such high grade images. This conventional technique is to detect images as follows. That is, while a broadband white light source is employed as a light source and focal points are defined at different places along a Z direction with respect to each of wavelength ranges of white light, two systems of image sensors are arranged in such a manner that the image sensors are focused onto both a surface layer and a rear plane of an object in the case that the object owns stepped portions. In two systems of these image sensors, focusing positions on the object planes are made different from each other along an optical axis direction in accordance with longitudinal chromatic aberration of an objective lens. As a consequence, images of different planes of the object are detected by the respective image sensors by utilizing the longitudinal chromatic aberration of the objective lens. It should be understood that as to a detecting optical path for two systems of the image sensors, band-pass filters are arranged in optical paths defined by that an optical path is branched and thereafter the branched optical paths are reached to the respective image sensors in such a manner that such light corresponding to the respective longitudinal chromatic aberration may penetrate through these band-bass filters.
The above-described conventional technique is directed to such a technical idea that images of different planes on a wafer are detected by employing two systems of the image sensors, and then images focused on the respective image sensors are employed. As a consequence, this conventional technique is not to produce a new image from the images detected by two systems of these image sensors, but is directed so as to select any one of the images detected from two systems of these image sensors so that this selected image is used in the defect inspection. However, in thin-film manufacturing processes typically known as semiconductors, flatting (planer) process operations are carried out as to wafer surfaces based upon the CMP (Chemical Mechanical Polishing) process operation. Thus, the thin-film manufacturing processes need not detect images at different heights on a wafer by employing the above-explained two image sensors. Also, even when stepped portions are formed on wafers, since structures of semiconductor logic products are complex, the selective use of such images detected by two systems of these image sensors cannot be employed.
To detect defects of very fine patterns in high precision, wavelengths of illumination light must be made shorter. Generally speaking, laser light sources are necessarily required in order to secure sufficiently large amounts of illumination light of light sources having short wavelengths for inspection purposes. However, in the case that such laser light sources are employed as illumination, interference problems of laser light may occurs. In other words, there are problems as to temporal/spatial coherence, problems of interference noises which are produced by thin-films formed on surfaces of samples, problems of contract between very fine patterns and background patterns, problems of illuminance fluctuations of pulse illumination light, and the like.
In accordance with the present invention, while a laser having a large light amount is employed, defects of patterns can be inspected in a high sensitivity by solving the above-described problems as to the temporal/spatial coherence occurred because of using the laser in the above-explained illumination light source.
In other words, the present invention is so arranged by that basic resolution of an optical system may be improved by shortening a wavelength of illumination light. The wavelength to be shortened is directed to DUV (Deep Ultra-Violet) light up to VUV (Vacuum Ultra-Violet) light. As a light source used in these wavelength ranges, there is an F2 laser (wavelength being 157 nm) as the VUV range. In order to employ these laser light as a defect inspecting optical system for illumination purposes, there are two technical aspects. As one technical aspect, brightness fluctuations of detected images and coherencies are reduced, which are caused in connection with film thickness fluctuations in optically transparent interlayer insulating films which are formed on a surface of an object. The brightness fluctuations caused by the film thickness fluctuations of the insulating films were reduced by employing such an arrangement that light having a plurality of wavelengths is illuminated. Also, a temporal coherence can be also reduced by illuminating the light having such plural wavelengths.
However, as to the light in the range from DUV to VUV, since a nitre material having high transmittance is restricted, for instance, in such a case that both the VUV light and the DUV light are coaxially illuminated, chromatic aberration cannot be corrected. As a consequence, while the light having the respective wavelengths is coaxially illuminated, such a chromatic aberration which cannot be corrected is detected in such a manner that a detection optical path is branched into two optical paths corresponding to the wavelengths, and then, image sensors are arranged on image planes of the respective wavelengths. As a result, as to an object plane (same plane) for a subject matter, two images are detected which are focused within two wavelength ranges. Since these two images are electrically synthesized with each other (namely, new image is produced by employing two images), it is possible to detect such an image having high resolution, from which noise could be reduced in view of defect inspection.
These and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings.